Google Maps
Google Maps forbids such uses. The only allowed access to their content is through their Maps API (see section 10.1 of the ToS).
Bing Maps Web Services
Bing Maps is the only commercial provider I know that allows such uses, through their Web Services feature. You will need an API key, though.
Bing will be a bit annoying to get, since they use quadkeys (an unique index for each tile) instead of {x,y} coordinates (good comparison there). You'll therefore need to create a new L.ILayer
implementation that conforms to this URL scheme.
Since you're of course not the first one to have this request ;) , some others have already coded such implementations. One is a pending pull request to the Leaflet repo, the other is directly available as a Gist. Try whichever you prefer, and add your weight to the pull request to make it merged faster :)